Transition alumina phases induced by heat treatment of boehmite: An X -ray diffraction and infrared spectroscopy study

During high temperature oxidation of alumina-forming alloys, various transition aluminas are formed before reaching the most stable α-Al 2O 3 phase. In particular, the γ, δ and θ transition aluminas are concerned. Precise studies on the development, the microstructure and the properties of the oxide layers are not straightforward owing to the difficulty to unambiguously distinguish the various polymorphs by X-ray diffraction (XRD) alone. To remove this difficulty, we propose a procedure which combines XRD and infrared (IR) spectroscopy. γ, δ, θ and α-Al 2O 3 phases, prepared by dehydration of aluminium oxihydroxide AlOOH (boehmite), were measured by XRD and IR spectroscopy on all samples. Thus, reference IR spectra were obtained for all alumina phases, as well as an assignment of the main band in agreement with previous studies, and coherent with the structural evolution of various polymorphs. This precise characterization may constitute a basis for further investigations on thin layers of alumina formed under various experimental conditions (temperature, atmosphere, etc.). Infrared spectra of alumina sequence γ→ δ→ θ→ α–Al 2O 3 obtained from 24h calcinations of boehmite at 873 K (a), 1123K (b), 1223 K (c), 1273 K (d), 1293 K (e), 1383 K (f), 1573 K (g).